Composite oxides involving barium titanate and strontium titanate or the like have a peculiar dielectric nature, a piezo-electric nature, a mechanical feature and a chemical feature, so that those composite oxides are a very important materials. These oxides have hitherto been manufactured by carrying out the reaction in the solid phase at a high temperature. For example, a titanate salt of an alkaline earth metal has been manufactured by a reaction step of a titanium oxide with a carbonate salt of an alkaline earth metals at a high temperature. The said reaction step is represented by the following reaction formula: EQU TiO.sub.2 +MCO.sub.3 .fwdarw.MTiO.sub.3 +CO.sub.2
(wherein M denotes an atom of an alkaline earth metal.) However, the composite oxides obtained by this process contain a comparatively large amount of impurity and further, as to reactants of the composition, for example, in the event of producing the titanate salt of alkaline earth metal, the reaction compound does not necessarily contain the stoichiometric amounts, even if the reaction is carried out with a definite condition by choosing a definite atomic ratio (Ti/M) of titanium (Ti) in proportion to the alkaline earth metal (M). Further, the composite oxides obtained have a granular diameter of several microns or more and a distribution of its granular diameter has a wider range. Consequently, the composite oxides obtained with this process can not be said to have a high efficient capacity as electronic raw material which requires a small size mechanical structure having the high capacity.
As a process for producing the composite oxides and improving the drawback of the solid phase reaction, a method for performing a chemical reaction in a solution (hereinafter, this is abbreviated as "the solution method") is proposed. Generally, the solution method has the following features.
(1) A purifying step by removing the impurity can be comparatively easily carried out and thereby, a high purity of the desired material is obtained.
(2) The desired material having a granular diameter of one milli-micron or less and a narrow distribution of granular size is obtained.
(3) An addition of a third material is easily carried out and even if a micro amount is added, a homogeneous composition can be obtained and consequently, the desired material having features conforming to the desired purpose can be obtained.
However, the process for producing the composite oxide with the conventional solution method involves various drawbacks, so that it can not be adopted as an industrial process of obtaining the composite oxide.
For example, in a process for producing a finely pulverized particles of barium titanate (BaTiO.sub.3) by a reaction of an aqueous solution containing barium hydroxide [Ba(OH.sub.2)] with titanium alkoxide [Ti(OR).sub.4 ] as disclosed in [J.A.C.S. 77 6194 (1954) and J.Amer.Ceram.Soc. 49 291 (1966)], it is difficult to control the ratio of the titanium/barium (Ti/Ba) in the finished product as a stoichiometric value.
In one process for hydrolyzing an alcohol mixed solution of barium alkoxides [Ba(OR).sub.2 ] and titanium alkoxides [Ti(OR).sub.4 ] and producing barium titanate [BaTiO.sub.3 ], as disclosed in the [U.S. Pat. No. 3,647,364] and in another process for producing strontium titanate by using strontium alkoxides [Sr(OR).sub.2 ] in the place of the barium alkoxides and the procedure disclosed in the [J.Amer.Ceram.Soc. 53 91 (1970)], the alkaline earth metal alkoxides used as the raw material are expensive and it is difficult to obtain the raw material. Therefore, these processes are not practical.
In the process for producing barium titanate [BaTiO.sub.3 ] with a reaction of barium hydroxide [Ba(OH).sub.2 ] and titanium tetrachloride [TiCl.sub.4 ] as disclosed in [U.S. Pat. No. 3,725,539] and in another process for producing barium titanate [BaTiO.sub.3 ] by treating barium chloride [BaCl.sub.2 ] and titanium tetrachloride [TiCl.sub.4 ] with hydrogen peroxide [H.sub.2 O.sub.2 ], as disclosed in Japanese Open Pat. No. 69599/1974, or in other words, in the process for producing barium titanate, by using a compound other than the titanium alkoxide, as a titanium source, it is difficult to control the ratio of titanium/barium as a stoichiometric quantity. Also, these reaction operations are complicated.
In a process for making barium titanate by the thermal decomposition of titanyl barium oxalate which is obtained by reacting with a mixed solution containing titanium ion and barium ion and oxalic acid, as disclosed in the [U.S. Pat. No. 2,758,911] the titanyl barium oxalate is thermally decomposed at a high temperature of 700.degree. C. or more and consequently, a coarse grain of the crystal is produced. Furthermore, it is difficult to add a third material.